In a radio communication apparatus, such as a mobile phone terminal, a modulation system in which the communication information is mapped on a so-called I-Q plane of a carrier for modulation for e.g. QPSK (quadrature phase shift keying), p/4 shift QPSK, 8PSK or HPSK (hybrid phase shift keying), is widely used. In such mobile phone terminal, an I-signal and a Q-signal are generated in a baseband side circuit and modulated by the quadrature modulator to generate a wireless signal.
Meanwhile, if a d.c. component is left in the I-signal or in the Q-signal, output from the baseband side circuit, or in the I-input circuit or in the Q-input circuit on the quadrature modulator side circuit, carrier components are superposed on the modulated wireless signal, thus increasing the so-called origin offset to deteriorate modulation accuracy or to increase errors on a demodulator side. The d.c. offset component is subjected to variations from one device to another, such that offset component is not reduced to zero. In case the d.c. component is left in the IQ input circuit of the quadrature modulator, the d.c. component is superimposed on the IQ input, thus generating unneeded signal called ‘carrier leak’. In case the carrier leak signal is larger than the signal component, the origin offset of the modulated output signal is increased to deteriorate the modulation accuracy (error vector magnitude: EVM) or to increase the demodulation error on the demodulator side. The offset component is subjected to variations from one device (e.g. a semiconductor device inclusive of the quadrature modulator) to another, such that the component is not reduced to zero. However, if the allowable offset value in e.g. a device is set to a smaller value, the production yield of the device is lowered due to offset by a test conducted at the time of manufacture.
Thus, in manufacturing a radio communication apparatus, it is necessary to provide a scheme of detecting and canceling out the residual offset component. This processing, termed ‘IQ signal offset adjustment’, is referred to as ‘IQ offset adjustment’ in the context of the present description. This processing is an indispensable process in the manufacture of a mobile phone terminal employing a quadrature modulator.
A typical example of the conventional IQ offset adjustment is now briefly explained. In the IQ offset adjustment, residual offsets of the I-signal and in the Q-signal are detected and intentionally applied as d.c. offset of the IQ signals to cancel out the residual offsets. Since the I-signal and the Q-signal are two-dimensional signals, it is difficult to adjust the I-signal and the Q-signal simultaneously. Hence, each of the I-signal and the Q-signal is adjusted at a time, by exploiting the orthogonality of I-signal and the Q-signal, in order to give an optimum offset cancellation value.
First, the I-offset is set to a certain fixed value, and the Q-offset is varied and several Q-offset values are measured to find the Q-offset value which will minimize the carrier leak. This Q-offset value is set as an optimum Q-side point. In similar manner, the Q-side offset is fixed at the optimum point thus found and the I-side offset is varied until such a point is found which will minimize the carrier leak. This point is set as an optimum I-side point.
The I-side is a gain fixed and the Q-side is changed to find an optimum point.
The above processing is repeated until the IQ offset value remains unchanged. This adjusts the IQ offset value.
This adjustment technique, which sequentially or empirically finds out the I-offset and the Q-offset, necessitates a large number of measurement and adjustment operations, whilst the time needed for adjusting a sole terminal (radio communication apparatus) is not unstable and becomes protracted.
The Patent Document 1, indicated below, discloses a configuration of arithmetically calculating the I-offset and the Q-offset to be corrected by measurement of several origin offset values. This method directly measures the origin offset value, instead of measuring the carrier offset value, such that one IQ offset pair needs to be set from time to time.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2000-124964A (pages 4 and 5, FIG. 4)